Fashion and Style are two completely different things. But many people do use the words interchangeably. When I was young my mother used to say that Fashion was all the clothes your mother or grandmother wore, and Style was what they did to make themselves look good. I disagree, and as I researched Fashion and Style more I came to the conclusion that both terms have more to do with the aesthetics of dressing than anything else.
Fashion actually refers to the general trends of dressing, and Style pertains to how a particular style is arrived at. So basically Fashion would be what popular clothes manufacturers sell to people and make it fashionable, while Style is how they arrive at that fashion. Fashion is pretty safe, especially when you understand little about Fashion. The reason being that as soon as something becomes Fashionable, it starts getting advertised and commercialized, which means if one company starts making clothes on a particular theme, all the other companies try to get in on the action by making their own clothing on that theme as well.
So, when something becomes Fashionable, it means that it has become popular among a large number of people, and this popularity can be measured by some sort of popular attention or sales. This means that if a dress gets enough publicity or sales, then that dress becomes a Fashionable item, and the most Popular dress becomes a staple in many wardrobes. Thus fashion is more to do with style than it is with making money, although Style is certainly important in helping to make money. And if a company wants to succeed in the world of fashion and style, they have to be very good at both Fashion and Style, as combining the two successfully is almost impossible.